I haven't run data using both providers. My experiences with Google are from semi-manually running maps in Google Maps and Google Earth.

Yahoo seems more forgiving and to try harder to return an approximate geocode. Also, since you get a "scope" out of the Yahoo API, it would return an area geocode (zip or city) when Google would just fail.

The age of the data might also be a factor: It was funny getting Google maps with properties that were over hills or parking lots. The geocode (Yahoo and/or Google's) would be accurate, but the map would be incorrect because the road data and satellite imagery hadn't been updated.